This invention relates to 2-phenylpyridine derivatives, and more particularly to novel liquid crystal compositions including 2-phenylpyridine derivatives for use as electro-optical display materials.
Liquid crystal display devices have many advantages. The liquid crystal display unit including the driving circuitry is small and flat. It can be driven at low voltages with low power consumption. The displays do not cause eye strain for the user since the liquid crystal display is a passive element. In view of these advantages, the liquid crystal display devices have become widely used in various display devices, such as watches, electronic portable calculators and the like. This is particularly true recently as people have concentrated their focus on liquid crystal display devices to provide character display or graphic display for use in computer terminals and POS terminals in place of conventional cathode ray tubes. Accordingly, there is strong demand for greater capacity liquid crystal display devices.
Given this background, the multiplex driving method has become the most widely used with twisted nematic liquid crystal display devices (hereinafter "TN cell"). The multiplex driving method has been developed to enhance the performance of such liquid crystal display devices.
It is well known that the performance of the TN cell driven by the multiplex driving mode largely depends on the dynamic characteristics of the liquid crystal material used therein. It is necessary that the liquid crystal have a liquid crystal temperature range wide enough to cover fully the temperature range within which the TN cell will be used. To provide excellent electro-optical response, the liquid crystal must be colorless, must be chemically inert and electrically stable against heat and light. When considering the electro-optical response, visual dependence and the voltage-brightness of the TN cell and the steepness of the threshhold are particularly significant properties.
There are many compounds which are colorless and have stable properties. However, few of the existing compounds provide the desired liquid crystal temperature range and electro-optical response. Accordingly, it has been necessary to mix at least two nematic liquid crystal compounds together with additional nonnematic liquid crystal compounds in order to provide a liquid crystal composition suitable for use in a liquid crystal display device.
In order to provide high-density multiplex driving of a TN cell the following factors in the voltage-light transmittance property of the TN cell as illustrated in FIG. 1 are particularly significant:
Viewing Angle Dependency .alpha.=V.sub.50 .multidot.O90.degree./V.sub.50 .multidot.O50.degree. PA1 Steepness of Response .beta.=V.sub.10 .multidot.O90.degree./V.sub.90 .multidot.O90.degree.
the smaller the above values of .alpha. and .beta. are, the more suitable the material is for use in the multiplex driving mode in a TN cell.
.alpha. and .beta. are closely related to the physical properties of the birefringence anisotropy (.DELTA.n) and the elastic constant K.sub.33 /K.sub.11 of the liquid crystal material in the cell. Generally, by reducing the value of the product (.DELTA.n.multidot.d of .DELTA.n and the thickness of the liquid crystal layer (d), .alpha. is reduced and the applicable viewing angle is increased. However, if .DELTA.n.multidot.d-1.0 .mu.m or less, the steepness .beta. deteriorates. Further, it is known that if .DELTA.n.multidot.d is constant, as K.sub.33 /K.sub.11 becomes small .beta. also becomes small. Based on this, B. S. Scheuble and G. Bauer in Japan Display '83, Proceedings of the 3rd International Display Research Conference, SID, p. 224, 1983, conducted measurement on known liquid crystal materials and identified the better performing liquid crystal compounds.
It is known that .alpha. and .beta. are influenced by the construction of the TN cell, such as the thickness of the cell (d) and the orientation of the panels therein. Moreover, when an improved device is required, a TN cell with improved properties is necessary. For example, if the display capacity is to be increased, a more steep threshhold property is required. Additionally, if the information display device is for out-door use, a much wider range of applicable viewing angle is necessary. Thus, at least an optimum value of the above-noted .DELTA.n.multidot.d varies with the requirements of the display characteristics. Accordingly, the physical properties of standardized liquid crystal materials are not always suitably used in various TN cells.
Based on this technical background, it is desirable to provide an improved liquid crystal material suitable for use in a TN cell driven in the multiplex driving mode. It is particularly desirable to provide liquid crystal compounds to provide a desired .alpha. and .beta. by mixing the materials with each other or with other liquid crystal materials for use in TN cells. Such liquid crystal materials must be colorless, stable in all respects, easily mixed with existing nematic liquid crystal compounds or compositions in order to improve the properties, such as the liquid crystal temperature range and the electro-optical response.